1. Field of the Invention
The present invention relates to microresonator packaging and tuning.
2. Description of the Prior Art
Microresonators of tuning fork configuration are disclosed in the inventor's copending U.S. application Ser. No. 122,313 now U.S. Pat. No. 3,683,213. The microresonators are fabricated of piezoelectric or ferroelectric material and have an overall length of from about 100 mils to 500 mils, an overall width of from about 15 mils to 50 mils, and a thickness of less than about 3 mils. Use of the tuning fork structure permits simple pedestal mounting. Typically, the oscillation frequency of these microresonators is in the frequency range of 10 kHz to 100 kHz with stabilities of 0 to 10 parts per million per degree centigrade.
Microresonators of the type described advantageously are employed as the time standard of a watch or clock. The small size permits the microresonator to be housed together with microelectronic oscillator, frequency divider and driver circuitry in a package sufficiently small to fit in a ladies wristwatch. The microresonators also are applicable in tone telemetry as for automatic meter reading, in filters for paging systems, and as time standards for data transmission systems, calculators and the like.
Microlithographic techniques useful for microresonator batch fabrication are described in the above mentioned U.S. patent application. One object of the present invention is to provide methods for packaging such microresonators on a mass production basis.
Although microresonators can be batch fabricated to frequencies relatively close to a desired value, it is necessary to tune each microresonator individually to obtain an exact frequency. Tuning is facilitated by providing on each tuning fork tine a thick metal film weight. Excess thick film material is removed, as by a laser, to achieve the desired mass and hence microresonator frequency. Another object of the present invention is to provide a technique for automatically accomplishing such tuning.
The microresonator frequency may change slightly during the packaging process as a result, e.g., of the heating required to bond the pedestal to a substrate or to complete hermetic sealing of the package. Mechanisms such as lattice dislocations may cause the stiffness of the piezoelectric material to increase over a period of time, with concomitant increase in microresonator frequency. Pre-aging of the tuning fork during manufacture minimizes subsequent frequency variation. These factors make it preferable to carry out the individual microresonator tuning after packaging and pre-aging have been completed. To this end, a further object of the present invention is to provide a housing which permits tuning of the microresonator after other production steps have been completed.
A microresonator used as the time standard in a wristwatch may increase in frequency slightly over an extended period of months or years. It is desirable to provide simple means for correcting this frequency deviations without modifying the tuning fork weights or replacing the microresonator itself. The necessary fine tuning can be accomplished by adjusting the value of a capacitor included in an oscillator circuit using the microresonator as a frequency source. Another object of the present invention is to provide a capacitor of novel configuration which uses the microresonator package as the capacitor dielectric, and which permits correction of the oscillator frequency by a simple operation performed external to the package.